Abstract: Slope stability of open-pit mines in cold regions is affected by adverse climate, engineering disturbance and other factors, which can easily induce geological disasters such as frost heave cracking, thaw subsidence and sliding collapse of rock masses. To explore the variation law of microscopic pore structure inside slope rock mass of open-pit mine under freeze-thaw action and reveal its freeze-thaw damage and deterioration mechanism, taking the mudstone from the slope of north Huolinhe Open-pit Mine in Inner Mongolia as the research object, freeze-thaw cycle tests of mudstone under different temperature gradients and different freeze-thaw cycle times were systematically carried out. Scanning Electron Microscope (SEM) was adopted to perform high-resolution scanning on internal fractures of mudstone to observe the morphological changes of microscopic pores inside mudstone. On this basis, advanced image processing technology was used to accurately extract and quantitatively count pore information from SEM images denoised by an optimization algorithm, and the pixel coverage method was applied to calculate the fractal dimension for characterizing the complexity of pore structure. The results show that: with the increase of freeze-thaw cycles and the decrease of freeze-thaw temperature, the smooth lamellar structures inside mudstone gradually decrease, while flaky and honeycomb deterioration structures increase significantly, presenting obvious freeze-thaw damage characteristics; the pore size distribution exhibits a trend of decreased average proportion of small pores and increased average proportion of medium and large pores, the average proportion of small pores in specimens decreases from 81.60% to 77.53%, the average proportion of medium pores rises from 17.84% to 21.90%, and the average proportion of large pores increases from 0.52% to 0.59%, which reflects the evolutionary process that small pores develop, penetrate under frost heave force and transit to larger pore sizes; the fractal dimension of pores obtained from SEM images of specimens shows a declining trend under multiple observation perspectives, indicating that freeze-thaw damage continues to intensify with the change of test conditions, and the overall pore structure evolves from complex to simple.